Vanillin (3-methoxy-4-hydroxybenzaldehyde) is the main component responsible for the olfactory and gustatory properties of the vanilla extract derived from Vanillia planifolia pods. It is one of the most used aromatic molecules in the industry. However, the production of natural vanillin from vanilla pods or vanilla extract only accounts for 20% of this market; its use is limited on the one hand because of the potential of available pods worldwide and on the other hand because of the widely fluctuating, high price of these pods (of the order of 30 £/kg to 450 £/kg i.e. a minimum of 1500 £/kg of natural vanillin potential).
Synthetic vanillin is therefore often used as an inexpensive substitute (approximately 15 £/kg) for natural vanillin for use in perfumery, cosmetics and in the food-processing industries. However, synthetic flavourings are less appreciated by consumers than natural flavourings.
It is for this reason that it is sought to obtain natural aromatic molecules, particularly vanillin, by biological processes, in particular bioconversion, which utilize microorganisms such as bacteria.
Within the meaning of the present invention, by bioconversion is meant the biological transformation of a substrate, preferably originating from a natural source, in order to obtain flavourings, fragrances or precursors of natural flavourings or fragrances.
Vanillin can be produced according to the following reaction diagram:

Several processes for producing natural molecules such as vanillin have already been described in the prior art.
In particular, the patent EP 0885968 describes a process for the production of vanillin in a bacterium belonging to the genus Streptomyces. This process comprises: a) the culture in a suitable medium of a bacterium belonging to the genus Streptomyces so as to form a fermentation broth, b) the addition of 5 to 40 gL-1 of ferulic acid to said broth in order to produce vanillin, and c) the extraction of the vanillin and guaiacol produced.
Another example described in the article by J. Overhage et al. discloses 1) the expression of the vanillyl-alcohol oxidase gene in Escherichia coli and 2) the expression of the genes calA (encoding coniferyl alcohol dehydrogenase) and calB (encoding coniferyl aldehyde dehydrogenase). This process would allow ferulic acid to be produced as the main product of the conversion of eugenol by the final recombinant strain E. coli XL1-Blue (pSKvaom-PcalAmcalB) (Highly efficient biotransformation of eugenol to ferulic acid and further conversion to vanillin in recombinant strains of Escherichia coli, 2003, Applied and Environmental Microbiology pp. 6569-6576). It is stated that the ferulic acid produced by this strain could act as a substrate for a second multi-recombinant E. coli strain (pSKechE/Hfcs) to produce natural vanillin (J. Overhage et al. Appl. Env. Microbiol. 65: 4837-4847, 1999).
A final example of the production of phenol derivatives from eugenol was described in the article “Harnessing eugenol as a substrate for production of aromatic compounds with recombinant strains of Amycolatopsis sp. HR167” (J Biotechnol. 2006 Sep. 18; 125(3):369-76. Epub 2006 May 4. Overhage J., Steinbuchel A., Priefert H.). This article discloses the expression of the vanillyl-alcohol oxidase gene in the Amycolatopsis sp. strain HR167, allowing the catalysis of the conversion of eugenol to coniferyl alcohol, coniferyl aldehyde, ferulic acid, guaiacol and vanillic acid.
These three examples of the production of natural vanillin precursors or natural vanillin itself have the drawback firstly of having a relatively high production cost due to the use of purified ferulic acid which is an expensive raw material, and secondly of being difficult to implement industrially due to 1) the use of non-food multi-recombinant strains and 2) the low yield of the bioconversions.
In order for a biological process using microorganisms to be profitable, it is preferable to use a widely available and very inexpensive substrate and a harmless and genetically stable microorganism. The inventors therefore sought to develop a process for the production of natural vanillin precursors or natural vanillin itself, which is simple to implement industrially, i.e. preferably comprising only one stage, and having a lower production cost than that of the prior art. The solution proposed by the invention is to use a low-cost and available natural substrate, which eugenol is, as well as a class 1 strain belonging to the genus Streptomyces, such as for example Streptomyces griseus. 